Designers and inventors have sought to develop a light modulator which can operate alone or together with other modulators. Such modulators should provide high operating speeds, a high contrast ratio or modulation depth, have optical flatness, be compatible with VLSI processing techniques, be easy to handle and be relatively low in cost. Two such related systems are found in U.S. Pat. No. 5,311,360 and 5,841,579 which are hereby incorporated by reference.
According to the teachings of the ‘360 and ‘579 patents, a diffractive light modulator is formed of a multiple mirrored-ribbon structure. An example of such a diffractive light modulator 10 is shown in FIG. 1. The diffractive light modulator 10 comprises elongated elements 12 suspended by first and second posts, 14 and 16, above a substrate 20. The substrate 20 comprises a conductor 18. In operation, the diffractive light modulator 10 operates to produce modulated light selected from a reflection mode and a diffraction mode.
FIGS. 2 and 3 illustrate a cross-section of the diffractive light modulator 10 in a reflection mode and a diffraction mode, respectively. The elongated elements 12 comprise a conducting and reflecting surface 22 and a resilient material 24. The substrate 20 comprises the conductor 18.
FIG. 2 depicts the diffractive light modulator 10 in the reflection mode. In the reflection mode, the conducting and reflecting surfaces 22 of the elongated elements 12 form a plane so that incident light I reflects from the elongated elements 12 to produce reflected light R.
FIG. 3 depicts the diffractive light modulator 10 in the diffraction mode. In the diffraction mode, voltage applied to alternate ones of the elongated elements 12 causes those alternating elongated elements 12 to move toward the substrate 20. The charged alternating elongated elements are referred to as the “active” elements. The voltage is applied between the reflecting and conducting surfaces 22 of the alternate active ones of the elongated elements 12 and the conductor 18. The voltage results in a height difference between the alternate active ones of the elongated elements 12 and noncharged or “bias” ones of the elongated elements 12. A height difference of a quarter wavelength λ/4 of the incident light I produces maximum diffracted light including plus one and minus one diffraction orders, D+1, and D−1.
FIGS. 2 and 3 depict the diffractive light modulator 10 in the reflection and diffraction modes, respectively. For a deflection of the alternate ones of the elongated elements 12 of less than a quarter wavelength λ/4, the incident light I both reflects and diffracts producing the reflected light R and the diffracted light including the plus one and minus one diffraction orders, D+1, and D−1. In other words, by deflecting the alternate ones of the elongated elements 12 less the quarter wavelength λ/4, the diffractive light modulator 10 produces a variable reflectivity.
FIG. 4 depicts a graphical representation of diffraction of 0th order light of the diffractive light modulator 10 at various active ribbon voltages with respect to intensity and attenuation. The active ribbon voltage 30 is graphically represented along the horizontal axis in Volts(V) and a normalized intensity 32 scale and attenuation 34 scale in decibels(dB) are shown along each vertical axis. Both the intensity graph 36 and the attenuation graph 38 have large negative slopes that decrease drastically as the active ribbon voltage 30 exceeds 10V. FIG. 4 is a representation of how voltage error, as will be discussed below, affects the performance of the diffractive light modulator 10.
Unfortunately, diffractive light modulators 10 are sensitive to voltage errors that may occur in normal operation. Specifically, there is a large dependence of attenuation with active ribbon voltage. This is particularly a problem at larger attenuations (−15 dB) where the dependence exceeds 10 dB per volt. Such error makes it extremely difficult to diffract the proper amount of light from the diffractive light modulator 10 in that the electrical bias applied to every other one of the elongated elements 12 will have an additional voltage component. This additional voltage will separate the active and bias elongated elements 12 more or less than the desired amount, thereby causing light diffraction inconsistent with the desired operation of the diffractive light modulator 10.
The non-linearity in the voltage versus attenuation behavior places severe design constraints on the voltage source. A stable, high bit-depth voltage supply or precision non-linear supply is required for control of the attenuation level. For attenuation applications, a key metric of performance is the slope of the attenuation versus voltage response, namely decibels per volt (dBN). A low and constant dBN is desired. FIG. 4 illustrates a less desirable performance—varying slope, and high slope at larger attenuation levels.
Lastly, current diffractive light modulators 10 are sensitive to voltage errors caused by power supply noise. Such error is often referred to as “ripple.” What is needed is a diffractive light modulator 10 that is less sensitive to ripple in order to diffract a correct amount of light